Node with 2 color & alpha options
For the record, there are a few different methods to combine alpha correctly, loosely summed up as:

Correlated geometry: Use an add. Examples are where you are certain the geometry doesn't occlude each other at the subpixel level.

Uncorrelated geometry: Use a probabilistic A+B-AB. Examples include two pieces of glass overlapping, or any element where the geometry overlaps another piece.

Further reading:
http://lists.openimageio.org/pipermail/oiio-dev-openimageio.org/2012-July/005642.html